The Bear Giant-Skipper genome suggests genetic adaptations to living inside yucca roots

Giant-Skippers (Megathymini) are unusual thick-bodied, moth-like butterflies whose caterpillars feed inside Yucca roots and Agave leaves. Giant-Skippers are attributed to the subfamily Hesperiinae and they are endemic to southern and mostly desert regions of the North American continent. To shed light on the genotypic determinants of their unusual phenotypic traits, we sequenced and annotated a draft genome of the largest Giant-Skipper species, the Bear (Megathymus ursus violae). The Bear skipper genome is the least heterozygous among sequenced Lepidoptera genomes, possibly due to much smaller population size and extensive inbreeding. Their lower heterozygosity helped us to obtain a high-quality genome with an N50 of 4.2 Mbp. The ~ 430 Mb genome encodes about 14000 proteins. Phylogenetic analysis supports placement of Giant-Skippers with Grass-Skippers (Hesperiinae). We find that proteins involved in odorant and taste sensing as well as in oxidative reactions have diverged significantly in Megathymus as compared to Lerema, another Grass-Skipper. In addition, the Giant-Skipper has lost several odorant and gustatory receptors and possesses many fewer (1/3-1/2 of other skippers) anti-oxidative enzymes. Such differences may be related to the unusual life style of Giant-Skippers: they do not feed as adults, and their caterpillars feed inside Yuccas and Agaves, which provide a source of antioxidants such as polyphenols.